1. Earth’s Early History
Section 19.3

2. The Early Earth
Earth formed through collision of a series of cosmic debris
4.2 billion years ago it cooled enough for rocks to form
Early atmosphere had little to no oxygen,
Largely CO2, water vapor and nitrogen
Very inhospitable!

3. The first organic molecules
It was shown by Miller and Urey in 1953 that Earth’s early primitive atmosphere was capable of creating organic molecules – they created 21 amino acids!
Although the composition for the Early Earth that they used was incorrect, but the general idea was correct

4. How do you go from organic molecules to living cells?
A lot remains unknown about this step
Evidence suggests 200 – 300 million years after the first rocks formed, cells similar to bacteria were common – how?
Scientists have shown that under the correct conditions, large organic molecules can form protenoid bubbles called microspheres
Not cells – but have some characteristics of cells
Thought to acquire characteristics of living cells 3.8 billion years ago

5. Where did DNA and RNA come from?
No one is sure, but a number of hypotheses exist
RNA is thought to be able to form from simple organic molecules
Based on experimental evidence
RNA is very versatile – lead to the RNA world hypothesis
RNA existed before DNA, and lead to direct DNA directed protein synthesis

6. Where did the Oxygen come from?
First fossils that resemble bacteria come from 3.5 billion years ago
Evolved in the absence of oxygen
2.2 billion years ago, photosynthetic organisms evolved
Change the chemistry of the atmosphere
Cooled the Earth
Oxygen reacted with Iron in the Oceans – precipitating out
Atmospheric composition changed over time – became toxic to many early organisms on Earth - adapt or die!

7. When did Eukaryotic cells originate?
Proposed that 2 billion years, some ancient prokaryotes began to evolve internal cell membranes
Endosymbiotic theory: prokaryotic cells entered these prokaryotes with an internal memberane
Intruders did not infect or be digested, instead they began living within larger cells
A symbiotic relationship developed between primitive eukaryotic cells and the prokaryotic cells within them
Proposed due to similarities identified between membranes of chloroplasts and mitochondria and prokaryotes

8. Endosymbiotic theory explained

9. Modern Evidence
DNA evidence shows that DNA from bacteria and DNA from mitochondria and chloroplasts are similar
All have ribosomes of a similar structure and size
They all reproduce by binary fission - division by mitosis
All together these features suggest a common ancestor!

10. Sexual reproduction
Sexual reproduction increases genetic variation and therefore sped yo evolutionary change
Asexual reproduction leads to a direct genetic copy
Genetic variation is restricted to mutations
In sexual reproduction, genetic material comes from two parents
Increased variation and increased chance of adapting to new or changing environmental conditions
Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction
Adaptive radiation lead to great diversity

11. Key points
Earth’s original atmosphere was a harsh environment that could not support life as we know it
The exact origin of life is unclear, but it is though to stem from the production of organic molecules such as proteins and RNA, leading to the formation of protenoid microspheres
Photosynthetic bacteria lead to the production of an oxygen rich environment 2.2 billion years ago
Eukaryotic cells developed through endosymbiotic theory – the ingestion of prokaryotes by a slightly altered prokaryote
Sexual reproduction has lead to increased amounts of genetic variation and evolution